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D.I.Y Damp Assessment Guide
Rising Damp - is ground moisture that creeps up through the pores
and capillaries of masonry structures. This moisture contains varying amounts of dissolved soluble minerals such as nitrates, chlorides, sulphates etc
commonly referred to as “salts”. This salt contamination creates a saline solution within and on the surface of masonry walls. Saline solutions are
a deterrent against the growth of mould and are hostile to mould spores. The capillary moisture will affect the full thickness of the wall – not
just the inside or outside. As the exposed masonry surfaces are subject to the surface evaporation of the capillary moisture surface salt deposits
will accumulate on the walls surface - efflorescence. As moisture contents vary the internal salt content will go through a wetting/drying cycle which
causes the salts to expand and contract within the wall. This expansion can cause the surface of the masonry to flake and fall off and the mortar to
become very powdery – sub-florescence. Rising damp is a natural phenomenon hence the requirement of a damp-proof course. When a damp-proof course is
either missing, failed or been improperly installed the dampness will continue to rise often to heights occasionally exceeding one metre. Damp proof
courses are installed to protect the full thickness of masonry walls it would be extremely rare to have “rising damp” affecting only the inside
and not the outside of a solid masonry wall. Similarly brick cavity walls would also have a damp-course extending through the full thickness of the
brick wall below the start of the brick cavity and a timber floor structure. It would also be extremely rare to have “rising damp”
affecting only the inside brick leaf and not the outside. The volume of capillary moisture is fairly minimal and would never be seen to run down the
surface of a damp wall similar to moisture running down the inside of windows. Rising damp does not smell – But rotting timbers supported by wet
masonry do!
Where the damp symptoms are confined to the internal faces only , (surface dampness), of exterior masonry walls the most likely causes are
Condensation and/or Sub-Floor Dampness.
Condensation - occurs when warm moist air comes in contact with cooler surfaces. The water vapour cools and condenses back
to a liquid state and can often leave “run” stains on painted wall surfaces. Water vapour cannot carry salts and condensation from it will support
the growth of moulds. Condensation occurs as a surface dampness only and often will affect one side only of a masonry wall especially when a heated
room is separated from the cooler outdoor surface by an un-insulated wall, ( similar to a single glazed window ).
Sub-Floor Dampness - can also cause problems not only with painted masonry surfaces but can cause serious decay of the
structural floor timbers. Suspended timber floors require sufficient air flow to prevent moisture building up from under the floor from the exposed
soil. Insufficient air movement allows the humidity to increase and the vapour will impregnate and raise the moisture content of floor boards, joists,
bearers and even wooden stumps. As the damp moist air rises to the underside of the floor boards it will spread horizontally under the whole floor
area. Masonry walls, internal as well as external, impede the sub-floor ventilation and can lead to moisture seeping up around the perimeter of the
timber floor, behind the skirting boards until it is able to evaporate from any exposed porous surface. “White set” plaster will absorb this moisture
will become soft and swollen – this may be mistaken for efflorescence cause by rising damp. Obvious signs are increased dampness higher up the walls
in the internal corners of rooms, blistering paint and cracks around the top of the skirting boards. Mould will often be evident as this moisture is
formed from condensation and does not contain ground salts. Exposed polished timber floors will often be swollen and “cupped” and will appear to have
a “rippled” appearance. Signs of black spots or stains around the nail heads will also indicate high humidity levels under the timber floor as the
saturated timbers cause the nails to rust.
Assuming that the damp areas are not related to leaking drainage, leaking shower recesses, plumbing fixtures and the like, materials or sheds
piled or built to “bridge” existing damp-coursing, the following symptoms may be determined by a quick visual inspection – (consider multiple causes).
Rising Damp - Surface salt deposits, flaking deteriorating brick or stonework, symptoms apparent on both surfaces of
the wall, (inside and out including brick cavity walls), and usually no mould due to the presence of salts.
Condensation – Mould on exposed wall surfaces, normally more concentrated along lower area corners, mould behind furniture,
bed-heads, paintings and wall hangings located on external walls. Surface run stains. Normally restricted to the internal face only of external
walls.
Sub-Floor Dampness - Blistering paint, soft swollen plaster surface, occasional mould, cracking along the tops of the skirting
boards, height of dampness increasing in internal corners, musty earthy damp smell evident, swollen rippled timber floor surface with accompanying rust
staining around nail heads. With external walls it is restricted to forming on the inside face only.
Suggested Remedial Works
Rising Damp - check that the damp-course has not been bridged by, garden beds, paving, or log piles etc
against the outside walls. Install a new damp-course.
Condensation – improve insulation, improve heating and air circulation, and increase ventilation when laundering, cooking,
washing and showering. If occurring in a bedroom, improve ventilation by leaving the door open and/or window slightly open. In an extreme case it may
require a ceiling fan to be run in the winter with the setting switched to create airflow within the bedroom.
Sub-Floor Dampness – check that external sub-floor wall vents are unobscured and sufficient in number - install more if
required, (check that the internal floor bearers are not at the same height as the vents restricting air movement). Consider installing a sub-floor
ventilation system.
PLEASE NOTE
A new damp-course will not cure a sub-floor dampness problem or condensation.
Removing damp plaster and re-plastering with render containing a salt and water retarding additive will reduce vapour saturation of the render from the
sub-floor area but will merely mask the problem.
Treating an inner skin only of cavity brickwork with a new damp-course indicates that the problem is probably not rising damp but a condensation
or sub-floor dampness problem.
Moisture meters will indicate not only the presence of moisture but the presence of hygroscopic salts – they do not differentiate between dampness
caused by condensation, sub-floor dampness or rising damp. THEY CANNOT BE USED BY THEMSELVES TO DETERMINE THAT A WALL HAS “RISING DAMP”.
The purpose of a damp course is to protect both the masonry and any structural timbers supported by that masonry – installing a chemical damp-course or
plastic membrane in a masonry wall behind skirting boards above a suspended timber floor would not be allowed or accepted in a newly constructed house.
Why would you accept it in your home?
Only an “Electro – Osmotic” damp-course will dry masonry below the level of installation
If you would like to download a PDF version of the D.I.Y Damp Assessment, then you can do so by clicking on the link below.
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